This is a 371 of PCT/IT96/00193 filed Oct. 21, 1996.
The invention relates to a method for obtaining an absorbable or bioresorbable material for use as a covering element in preventing scar adherences.
More in detail, the present invention relates to a method for obtaining a material, provided with relatively short absorption times for the human body, for use as a covering element in a zone where scar adherences might develop, for example in the area around a suture performed on a tendon and/or a nerve tract, as well as internally of the spinal foramen after spinal surgery.
The invention further relates both to the material obtained by the method and to a cladding element for prevention of adherences realised with said material.
The invention finds its principal application in the medical-surgical field.
The most frequent complication consequent to surgical reconstruction of a tendon and/or a traumatically severed nervous tract is constituted by the formation of scar tissue adherences of the tendon suture line (or of the suture lines of the single nerves belonging to a bundle), with the surrounding soft tissues or the bone surface adjacent to the sutured lesion.
These adherences, which can occur independently of the surgical technique employed in repairing the lesion, can to a considerable degree limit the tendon sliding function, so much so that it is common practice to follow a first surgical operation with a second remedial operation, once the tendon has re-established its biological continuity, to improve tendon excursion: this is particularly common in the field of hand surgery with regard to the flexor apparatus of the hand.
Further, following spinal surgery operations, once the spinal foramen has been opened to afford access to the discs or nervous structures of the rachis, scar tissue forms almost constantly between the osteo-articular surfaces, the muscles around the vertebrae, the endocranial sac and the nerve roots, which limit the small movement that the nerve roots make inside the connecting foramen in order to adapt to the various positions assumed by the spinal column. This movement limitation can lead to painful clinical symptoms which might even be worse than those presented pre-operation and at the origin of the whole intervention.
To obviate the above-mentioned problem, apart from rendering interventions as non-invasive as possible, a practice often employed is that of isolating the nervous structures with free strands of fatty tissue removed from the sottocutaneous area; absorption of the fatty tissue is not, however controllable, and manipulation of same presents some difficulties.
In the field of regeneration of single lacerated nerves, use is made of generally tubular elements acting as guides for the two extremities of a nerve during the regeneration period thereof.
In this context, use is made of nerve guides constituted by non-biodegradable and bioresorbable materials, such as for example silicone rubber (G. Lundborg et al., Exp Neurol. 76 (1982) 361, G. Lundborg et al., Scand. J. Plastic Reconstructive Hand-Surgery 25(1991) 79, G. Lundborg et al. J Neuropathol Exp. Neurol. 41 (1982) 412, M. Merle et al., Microsurg. 10 (1989) 130, B. R. Seckel et al. Plastic Reconstructive Surg. 78 (1986) 793), Acrylic Polymers (B. G. Uzman and G. M. Villegas, J. Neurosci. Res. 9(1993)157), polyethylene (P. G. Cordeiro et al., Plastic Reconstructive Surg. 83 (1989) 1013) elastomer hydrogel (R. D. Keeley et al., J. Reconstructive Microsurg. 7(1991) (2) 93) or porous stainless steel (W. E. Kuhn and J. L. Hall in xe2x80x9cModern Developments in Powder Metallurgyxe2x80x9d, edited by H. H. Hausener and P. W. Taubenblat, American Powder Metallurgy Institute, Princeton N.J. p. 279).
The nerve guides, which are synthesised in a non-biodegradable material that remains in situ as a foreign body, can limit the regenerative function of the nerve and give rise to irritation, sometimes years after implantation, in some cases so serious as to necessitate a further operation to remove them.
Also known, in the field of specific application as nerve guides, is the use of biodegradable and bioresorbable material in a predetermined time period, in general comprised between six months and two years.
For example, the prior art teaches use of a copolymer derived from L-lactide combined with poly-xcex5-caprolactone (W. F. A. Den Dunnen et al. in xe2x80x9cA new PLLA/PCL copolymer for nerve regenerationxe2x80x9d, Journal of Materials Science Materials in Medicine 4 (1993) 521-525), G. Perego et al. in xe2x80x9cPreparation of a new nerve guide, from a poly(L-lactide-co-6-caprolactone)xe2x80x9d, Biomaterials 1994, Vol 15 no. 3, 189-193); with like aims an ester of hyaluronic acid (Favaro et al. in xe2x80x9cPeripheral Nerve Regeneration Through a Novel Bioresorbable Nerve Guidexe2x80x9d, ASAIO Transactions 1990 36 (3), M291-M294).
All of the above-mentioned materials have good non-toxic characteristics and low rejection reactions; reabsorption times are relatively long, generally over six months, for which reason these materials are not suitable for other applications requiring much shorter reabsorption times, for example one month.
xe2x80x9cAn Experimental Study on an Adhesions-Blocking Membrane in the Flexor Tendon of Chicken-Partxe2x80x9d, Clinical materials 6(1) 1-12 (1990) proposed the use of a membrane realised with a copolymer of leucine-polyeurethane blocks, derived from poly(tetra glycol methylene), toluene diisocyanate and hydrazine.
Copolymers containing 70% leucine were biodegradable in vivo (rat) in about six months, differently to those containing leucine 50%, which were non-biodegradable. From the published data it may be derived that an increase in leucine concentration leads to lower material elasticity and permeability.
For research into adherence inhibition in tendon regeneration it would seem that the 70% leucine composition was chosen as it offered a reasonable compromise between the needs for elasticity, permeability and biodegradability.
It must be remembered however that this material can generate, during degradation, an aromatic amine (toluene diamine), whose toxicity cannot be ignored.
Document U.S. Pat. No. 5350573 describes a method and composition for preventing adhesions during surgical operations. The surface of the tissues and the surgical instruments involved in the operation are covered with a solution of a water-wettable polymer before contacting tissue duringthe operation. The composition comprises a solution of polymer material having a molecular weight of 500000 or above with a weight concentration comprised between 0.01 and about 15%.
Finally, document U.S. Pat. No. 5358973 describes a composition which can be used in surgical operations with the aim of preventing adhesions between tissue surfaces, said composition being an aqueous solution containing dextran and hyaluronic acid.